This invention relates to a projection lens suited for use with apparatus using light of different wave lengths as seen in the process for producing IC (integrated circuits) by printing patterns of photo-masks in a silicon substrate wherein the wave length of the ray used for printing is different from that used for viewing the projected image.
In the process for producing IC, there is a step in which desired parts of the oxide layer formed on the silicon substrate are removed in order to diffuse impurities into the interior of the silicon substrate therefrom. The removal of the desired parts of the oxide layer is achieved by photoetching after a pattern corresponding to the desired parts has been printed through the photomask on a photo-sensitive coating (photoresist) applied on the substrate, and which can be made soluble or insoluble in the etching solution by irradiation of rays of light. A number of printing followed by etching cycles are superimposingly applied on a piece of IC with replacing of one mask by another, whereby the mask pattern which has been already printed and the next mask pattern which is to be printed should be precisely positioned relative to each other. This adjustment of the positions relative each other is termed "alignment," which is usually performed with viewing through a microscope.
The photoresist is generally sensitive to light of short wave lengths, while it is unsensitive to light of long wave lengths. Speaking in terms of the spectrum of a mercury arc lamp, which is usually used as a source of light for the printing, the photoresist is sensitive to the 365, 405 and 436 nm. lines, but insensitive to the 456 and 578 nm. lines.
In case printing is carried out by projection, it is usual to perform the alignment by use of the rays of light passing through the projection lens, so that the chromatic aberrations of the projection lens should be corrected for each of the wave lengths of both the alignment light and printing light. If the axial chromatic aberration is present, the image position of the mask pattern is shifted in the direction of the optical axis by an interchange of rays of light between the alignment and the printing, so that each time the alignment operation is changed over to the printing operation, there arises the difficulty that the mask or the substrate must be moved over again or shifted so as to be in focus. On the other hand, the presence of lateral chromatic aberration leads to an unfavorable result that, even if the alignment made by viewing is perfect, the printing will be made out of alignment.